Physics

Electrical Machines and Circuits

720 Questions

Electrical machines and circuits involve the principles of alternating and direct current, transformers, diodes, and power consumption. These concepts are essential for engineering and technical exams. Review these practice questions to test your knowledge of circuit analysis.

Transformers and impedanceAlternating and direct currentCircuit components and diodesPower consumption calculations

Electrical Machines and Circuits Questions

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A step-up transformer increases the AC voltage. The statement is 

  1. true

  2. false

  3. ambiguous

  4. data insufficient

Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A step up transformer increases the voltage This transformer has more windings on the "secondary" side of the transformer, with fewer windings of thicker wire on the primary or input coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

State whether true or false:

Transformers can change high voltage to low voltage.

  1. True

  2. False
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A transformer takes in electricity at a higher voltage and lets it run through lots of coils wounded around an iron core. Because the current is alternating, the magnetism in the core is also alternating. Also around the core, there is an output wire with fewer coils. The magnetism changing back and forth creates a current in the wire. Having fewer coils means less voltage. So the voltage is "stepped-down."

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The transformer is used in ................. current circuits.

  1. alternating

  2. directing

  3. no

  4. less
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A transformer consists of two coils - the primary coil and the secondary coil both wound on a soft iron core. If an ac voltage is applied to the primary coil then this will produce a changing magnetic field in the iron core. This induces an alternating voltage in the secondary coil according to electromagnetic induction.
A voltage in the secondary coil occurs only if the magnetic field is changing. Therefore, transformers will only work with ac current only. The transformer is used in alternating current circuits.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformers are used

  1. in DC circuit only

  2. in AC circuits only

  3. in both DC and AC circuits

  4. neither in DC nor in AC circuits
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformers work on the principle of voltage induction by the linkage of changing magnetic flux between between the two cores. Changing magnetic flux requires changing/alternating current. 
Transformers are used only in AC circuits. In DC circuits, no alternating/changing magnetic flux is produced to induce voltage in secondary from the primary terminals of the transformers.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

How will you  relate the number of turns in the primary coil of a step-down transformer  to that in secondary coil ?

  1. has large number of turns

  2. is very heavily insulated

  3. is very thin

  4. all the above
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

A step down transformer has more turns of wire on the primary coil and less turns of wire on the secondary coil. This makes a smaller induced voltage in the secondary coil. 

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

In a step down transformer, the number of turns in the primary is always :

  1. greater than the number of turns in the secondary

  2. less than the number of turns in the secondary

  3. equal to the number of turns in the secondary

  4. either greater than or less than the number of turns in the secondary
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

In step down transformers, voltage in secondary coil decreases. and we know that 


$\dfrac { { E } _{ s } }{ { E } _{ p } } =\dfrac { { n } _{ s } }{ { n } _{ p } } $

So, number of turns decreases in secondary coil with respect to primary coil.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer changes the voltage

  1. without changing the current and frequency

  2. without changing the current but changes the

    frequency

  3. without changing the frequency but changes the

    current

  4. without changing the frequency as well as the

    current
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

A transformers changes an alternating potential difference from one value to another of greater or smaller value using the principle of mutual induction. so, in transformer current and voltage changes but frequency remains same.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Statement A: In a transformer, a large alternating current at low voltage can be transformed into a small alternating current at high voltage
Statement B: Energy in current carrying coil is stored in the form of magnetic field.

  1. A is true but B is false

  2. Both A and B are true

  3. A is false but B is true

  4. Both A and B are false
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformer is used to increase voltage and decrease current in the secondary coil with respect to primary coil or vice versa.
Energy in the coil is always stored in the form of magnetic field.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

Transformers are used in

  1. d.c circuits only

  2. a.c. circuits only

  3. Both a.c and d.c circuits

  4. Integrated circuits
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Transformers are used only in A.C. Circuits because D.C. current could not produce changing flux which in turn not able to produce induced emf in the secondary coil of the transformer but A.C. Current produces changing flux.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A transformer works on the principle of :

  1. Self induction

  2. Mutual induction

  3. Coulomb law

  4. Ampere law
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

The Voltage Transformer can be thought of as an electrical component rather than an electronic component. A transformer basically is very simple static (or stationary) electro-magnetic passive electrical device that works on the principle of Faraday’s law of induction by converting electrical energy from one value to another.

The transformer does this by linking together two or more electrical circuits using a common oscillating magnetic circuit which is produced by the transformer itself. A transformer operates on the principals of “electromagnetic induction”, in the form of  $Mutual$ $ Induction$.

Mutual induction is the process by which a coil of wire magnetically induces a voltage into another coil located in close proximity to it. Then we can say that transformers work in the “magnetic domain”, and transformers get their name from the fact that they “transform” one voltage or current level into another.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

An ideal transformer has $500$ and $5000$ turns in primary and secondary winding respectively. If the primary coil is connected to a $6 V$ battery then the secondary voltage is

  1. $60 V$

  2. Zero
  3. $0.6V$
  4. $6.0V$
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.

A transformer takes high-voltage electricity with a small current and changes it into low-voltage electricity with a large current, or vice versa. That is, step up or step down the voltage in a circuit. A step-up transformer turns low-voltage electricity into high-voltage electricity while dropping the current. A step-down transformer changes high-voltage electricity into low-voltage electricity.
If we change the number of turns in the coils we change the induced emf. This allows us to change (transform) the voltage from the primary to the secondary coil.
The Turns Rule is:
Ns/Np=Vs/Vp
Where,
Ns = number of turns on the secondary coil
Np = number of turns on the primary coil
Vs = voltage across the secondary coil
Vp = voltage across the primary coil
So if number of turns on the secondary coil is more than on the primary coil, the output voltage will be higher than the input voltage. This is called a step up transformer.
Hence, The secondary voltage as per the given question can be obtained as follows:
Let input voltage be 6V and the output voltage be x. Let 5000 be the number of turns of secondary coil and 500 be the number of turns of primary coil.
Ns/Np=Vs/Vp; 
That is, 5000/500=x/6 
$x=\frac { 5000\times 6 }{ 500 } =60\quad V$.
Therefore, the secondary voltage is 60 V.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

A step down transformer reduces $220V$ to $11V$. The primary coil draws $5 A$ current and secondary coil supplies $90A$. Efficiency of the transformer will be: 

  1. $4.4\%$
  2. $20\%$
  3. $33\%$
  4. $90\%$
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

Answer is D.

The efficiency of a transformer can be calculated using Equation,
$Efficiency\quad =\quad \frac { Output\quad power\quad of\quad the\quad transformer }{ Input\quad power\quad of\quad the\quad transformer } \quad =\quad \frac { { P } _{ s } }{ { P } _{ p } } \times 100$
In this case, the output power is calculated as follows.
The output voltage is 11 V and current is 90 A
$Power\quad =\quad VI\quad =\quad 11\times 90=990W$.
The input power is calculated as follows.
The input voltage is 220 V and current is 5 A
$Power\quad =\quad VI\quad =\quad 220\times 5=1100W$.
So, $\eta =\quad \frac { 990 }{ 1100 } \times 100\quad =\quad 90%$.
The efficiency of the transformer is 90%.

Multiple choice physics electro-magnetism applications of electromagnets transformer transformers

The principle on which transformer works is?

  1. Mutual induction

  2. Self induction

  3. Electromagnetic induction

  4. None of these
Reveal answer Fill a bubble to check yourself
A Correct answer
Explanation

Answer is A.
Mutual inductance is where the magnetic field generated by a coil of wire induces voltage in an adjacent coil of wire. A transformer is a device constructed of two or more coils in close proximity to each other, with the  purpose of creating a condition of mutual inductance between the coils.